Added 'bumblebee', a package that facilitates NVidia Optimus graphics with X11.

Note: it relies heavily on 'virtualgl'. This also makes the approach taken
by bumblebee not very effective.
So, this package is actually mainly useful for shutting down your card so that
it does not consume power/produce heat.

See the comments in bumblebee/default.nix

svn path=/nixpkgs/trunk/; revision=32036